For vehicular ultrasound devices of the related art, as shown in FIG. 1, a transducer converts an electric signal generated by a pulse generation device into ultrasonic waves and transmits the ultrasonic waves, and the ultrasonic waves transmitted by the transducer are reflected by a surface of an object and then converted back by the transducer into an electric signal.
The electric signal, which has a level of a few microvolts (uV), obtained through the conversion by the transducer is amplified by an amplifier (Amp) to a signal having a level of a few volts (V) before being converted into a digital signal. However, since a vehicular ultrasonic wave signal has a frequency ranging from 20 kHz to 100 kHz, which is equal to or greater than an audio frequency, the amplified signal is filtered by a band-pass filter to increase a signal-to-noise ratio of a corresponding frequency band.
Subsequently, an envelope detector calculates an envelope of the filtered signal, and a ToF calculation and a comparator compare a signal value of the calculated envelope with a threshold.
If a result of the comparison is that the signal value of the envelope is greater than the threshold, it is determined that there is an object. In this case, the ToF calculation calculates a distance from the object using a speed of the ultrasonic waves and a time taken until an envelope signal equal to or greater than a threshold is received after a transmission time.
However, as described above, the vehicular ultrasonic wave signal is in a frequency band adjacent to an audible frequency band and is subject to interference caused by harmonic components of noise in the general environment.
Accordingly, when a level of the harmonic components of ambient environmental noise is equal to or greater than a threshold, the noise may be recognized and detected as an object although there is no object.
In order to reduce such a false alarm, vehicular ultrasound devices of the related art pass through two sensing processes to determine that there is an object only when two pieces of sensing information are the same.
That is, vehicular ultrasound devices of the related art should sense a road curb of a height that may damage a vehicle bumper. However, as shown in FIG. 3, since a road curb positioned in a short distance lies outside a vertical beam pattern, signal attenuation is great, and thus an amplifier having a high gain should be used to detect an object.
When an amplifier having a high gain is used, a first transmission signal is reflected by an object having a high reflection signal such as a wall positioned at a remote distance and received at a threshold or higher when a second transmission signal is transmitted or received.
In other words, as shown in FIG. 4, since time of flight (TOF) 1 of the first transmission signal is not equal to TOF 2 of the second transmission signal, an alarm may not be provided although there is an object.
In addition, if a vehicle is started up when an outdoor temperature is low, an air temperature layer may be formed by an exhaust gas which has a temperature higher than the outdoor temperature. In this case, as shown in FIG. 5, vehicle ultrasound devices of the related art amplify an ultrasonic wave signal reflected by the air temperature layer with a high gain.
Accordingly, when the ultrasonic wave signal reflected by the air temperature layer is equal to or greater than a threshold, the ultrasonic wave signal may be misrecognized as an object.